Solid electrolytic capacitor

ABSTRACT

The solid electrolytic capacitor of the invention contains an anode, a dielectric film formed on the anode by anodic oxidation, a solid electrolytic layer formed on the dielectric film, and a cathode connected with the solid electrolytic layer, the solid electrolytic layer being made from a conducting polymer obtained by doping with alkyl sulfonic acid ions, a polymer including a heterocyclic monomer unit represented by the following general formula as a repeating unit:                    
     The solid electrolytic capacitor of the invention has large capacitance and exhibits low impedance in a high frequency region.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/JP99/05867 which has an Internationalfiling date of Oct. 22, 1999, which designated the United States ofAmerica.

TECHNICAL FIELD

The present invention relates to a solid electrolytic capacitorcomprising a solid electrolytic layer of a conducting polymer formed ona dielectric film, and in particular, it relates to improvement of thesolid electrolytic layer for the purpose of providing a solidelectrolytic capacitor having large capacitance and exhibiting lowimpedance in a high frequency region.

BACKGROUND ART

In accordance with recent trend in reducing the size and weight ofelectronic equipment, there are increasing demands for a compactcapacitor having large capacitance and exhibiting low impedance in ahigh frequency region.

Capacitors conventionally used in a high frequency region are a plasticcapacitor, a mica capacitor and a laminated ceramic capacitor, all ofwhich are large in size and have a capacitance difficult to increase.

As a capacitor having large capacitance, an electrolytic capacitor iswell known. Examples of the electrolytic capacitor are an electrolyticcapacitor using an electrolyte (such as an aluminum electrolyticcapacitor) and a solid electrolytic

Recently, a conducting polymer prepared by doping a polymer, such aspolypyrrole and polythiophene, which is obtained by electrolyticallypolymerizing a heterocyclic monomer such as pyrrole or thiophene, withBF₄ ⁻ (borofluoride ions) or C10₄ ⁻ (perchlorate ions) has been proposedas a solid electrolyte usable instead of manganese dioxide (JapaneseLaid-Open Patent Publication No. 60-37114/1985).

The conducting polymer obtained by using the aforementioned halogenanions as a dopant, however, can easily degrade a dielectric film. Inaddition, the conducting property of the conducting polymer can beeasily lowered because it is poor in thermal stability and can be easilyundoped. The conducting polymer can be easily undoped particularly infixing an electrode at a high temperature of approximately 200° C.

The degradation of the dielectric film and the lowering of theconducting property of the conducting polymer can increase a leakagecurrent in a capacitor, resulting in reducing the capacitance andincreasing the impedance.

A conducting polymer obtained by using aryl sulfonic acid ions such asdodecylbenzene sulfonic acid ions and naphthalene sulfonic acid ions, asa dopant has been proposed for overcoming the aforementioneddisadvantages (Japanese Laid-Open Patent Publication No. 64-49211/1989).

The conducting polymer obtained by using aryl sulfonic acid ions as adopant has still rather high electric resistance although the electricresistance is lower than that of the conducting polymer obtained byusing BF₄ ⁻ or C10₄ ⁻ as a dopant. Therefore, even when this conductingpolymer is used as a solid electrolyte, it is difficult to obtain asolid electrolytic capacitor exhibiting low impedance in a highfrequency region.

The present invention was devised in view of these conventionaldisadvantages, and a main object of the invention is providing a solidelectrolytic capacitor having large capacitance and exhibiting lowimpedance in a high frequency region.

DISCLOSURE OF THE INVENTION

A solid electrolytic capacitor of the invention (hereinafter referred toas a first capacitor) comprises an anode, a dielectric film formed onthe anode by anodic oxidation, a solid electrolytic layer formed on thedielectric film made from a conducting polymer produced by doping apolymer including a heterocyclic monomer unit represented by thefollowing general formula as a repeating unit with alkyl sulfonic acidions and sulfuric acid ions, and a cathode connected with the solidelectrolytic layer.

Another solid electrolytic capacitor of the invention (hereinafterreferred to as a second capacitor) comprises an anode, a dielectric filmformed on the anode by anodic oxidation, a solid electrolytic layerformed on the dielectric film made from a conducting polymer obtained bydoping a polymer including a heterocyclic monomer unit represented bythe following general formula as a repeating unit with alkyl sulfonicacid ions in a ratio of one of the alkyl sulfonic acid ions per 2through 5 heterocyclic monomer units represented by the general formula,and a cathode connected with the solid electrolytic layer. In thisspecification, the first capacitor and the second capacitor may begenerically referred to as the capacitors of the invention.

wherein R¹ and R² independently indicate an alkyl group or H, and X is Sor NR³ (wherein R³ indicates an alkyl group or H).

The solid electrolytic layer is preferably formed by electrolyticpolymerization using, as a solution for electrolytic polymerization, anaqueous solution including a heterocyclic monomer such as pyrrole orthiophene and an ionic dopant such as alkyl sulfonic acid ions, becausethe dopant can be uniformly doped in the polymer in this manner.

The conducting polymer used in the first capacitor is preferably dopedin a ratio of one ion of the dopant per 2 through 5 heterocyclic monomerunits each represented by the above-described formula. The molar ratiobetween sulfuric acid ions and alkyl sulfonic acid ions used for dopingin electrolytic polymerization is not specified. It is, however,confirmed through experiments that the maximum molar ratio is 10:1. Thesulfuric acid ions cannot be doped exceeding this molar ratio. ions suchas dodecylbenzene sulfonic acid ions and naphthalene sulfonic acid ions,as a dopant has been proposed for overcoming the aforementioneddisadvantages (Japanese Laid-Open Patent Publication No. 64-49211/1989).

The conducting polymer obtained by using aryl sulfonic acid ions as adopant has still rather high electric resistance although the electricresistance is lower than that of the conducting polymer obtained byusing BF₄ ⁻ and C10₄ ⁻ as a dopant. Therefore, even when this conductingpolymer is used as a solid electrolyte, it is difficult to obtain asolid electrolytic capacitor exhibiting low impedance in a highfrequency region.

The present invention was devised in view of these conventionaldisadvantages, and a main object of the invention is providing a solidelectrolytic capacitor having large capacitance and exhibiting lowimpedance in a high frequency region.

DISCLOSURE OF THE INVENTION

A solid electrolytic capacitor of the invention (hereinafter referred toas a first capacitor) comprises an anode, a dielectric film formed onthe anode by anodic oxidation, a solid electrolytic layer formed on thedielectric film made from a conducting polymer produced by doping apolymer including a heterocyclic monomer unit represented by thefollowing general formula as a repeating unit with alkyl sulfonic acidions and sulfuric acid ions, and a cathode connected with the solidelectrolytic layer. Another solid electrolytic capacitor of theinvention (hereinafter referred to as a second capacitor) comprises ananode, a dielectric film formed on the anode by anodic oxidation, asolid electrolytic layer formed on the dielectric film made from aconducting polymer obtained by doping a polymer including a heterocyclicmonomer unit represented by the following general formula as a repeatingunit with alkyl sulfonic acid ions, and a cathode connected with thesolid electrolytic layer. In this specification, the first capacitor andthe second capacitor may be generically referred to as the capacitors ofthe invention.

wherein R¹ and R² independently indicate an alkyl group or H, and X is Sor NR³ (wherein R³ indicates an alkyl group or H).

The solid electrolytic layer is preferably formed by electrolyticpolymerization using, as a solution for electrolytic polymerization, anaqueous solution including a heterocyclic monomer such as pyrrole orthiophene and an ionic dopant such as alkyl sulfonic acid ions, becausethe dopant can be uniformly doped in the polymer in this manner.

The conducting polymer is preferably doped in a ratio of one ion of thedopant per 2 through 5 heterocyclic monomer units each represented bythe above-described formula. The molar ratio between sulfuric acid ionsand alkyl sulfonic acid ions used for doping in electrolyticpolymerization is not specified. It is, however, confirmed throughexperiments that the maximum molar ratio is 10:1. The sulfuric acid ionscannot be doped exceeding this molar ratio.

It is preferable to use aluminum or tantalum as an anode materialbecause a dielectric obtained by anodic oxidation provides a highdielectric constant and a high electrical insulating property. Since thedielectric film is formed by the anodic oxidation, when the anode ismade from aluminum or tantalum, the dielectric film is formed from anoxide of aluminum or tantalum.

The dopants used in the fabrication of the capacitors of the inventionare not halogen anions such as BF₄ ⁻ and C10₄ ⁻ but are non-halogenanions, and hence, the dielectric film is difficult to degrade. Also,the alkyl sulfonic acid ions exhibit higher thermal stability and aremore difficult to undope as compared with BF₄ ⁻ and C10₄ ⁻ even whenthey are exposed to a high temperature, for instance, in fixing anelectrode. Therefore, the electric conductivity of the conductingpolymer is not easily lowered. For these reasons, in the capacitors ofthe invention the leakage current is small and the deterioration ofproperties is small even when they are exposed to a high temperature.Moreover, since the electric resistance of the conducting polymer dopedwith alkyl sulfonic acid ions is lower than that of the conductingpolymer doped with aryl sulfonic acid ions, the capacitors of theinvention exhibit lower impedance and better frequency characteristic ascompared with the solid electrolytic capacitor disclosed in JapaneseLaid-Open Patent Publication No. 64-49211/1989.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view of a solid electrolytic capacitor fabricatedin Examples.

PREFERRED EMBODIMENT

While the invention is described in further detail below based on itsembodiment, the invention is in no way limited to the followingembodiment but may be carried out in a properly modified fashion as longas such a modification does not result in a substantial alteration ofthe spirit of the present invention.

EXAMPLE 1

A porous sintered tantalum was subjected to anodic oxidation, therebyforming a dielectric film made from tantalum oxide on the surfacethereof. Then, the sintered tantalum bearing the dielectric film thereonwas immersed in an aqueous solution containing 20 wt. % of hydrogenperoxide (oxidizing agent) and 1 wt. % of sulfuric acid for 30 minutes,and dried for 5 minutes at room temperature. The sintered tantalum wasthen exposed to pyrrole vapor for 30 minutes, thereby forming apolypyrrole film (vapor phase polymerized film) on the dielectric film.Since this polypyrrole film is doped with sulfuric acid ions, thepolypyrrole film is provided with a conducting property.

Subsequently, the sintered tantalum bearing the dielectric film on whichthe polypyrrole film had been formed was immersed in a solution forelectrolytic polymerization (an aqueous solution containing 0. 2M ofpyrrole, 0.06M of sodium dodecylsulfonate and 0.0003M of sulfuric acid).Then, with a stainless steel lead (anode terminal) attached to theaforementioned polypyrrole film, electrolytic polymerization(polymerization by electrolytic oxidation) was performed at a constantcurrent to form a solid electrolytic layer on the polypyrrole film, thesolid electrolytic layer being made from a conducting polymer obtainedby doping polypyrrole with dodecyl sulfonic acid ions and sulfuric acidions. Also as a counter electrode (cathode), a stainless steel electrodewas used.

Subsequently, the sintered tantalum bearing the polypyrrole film onwhich the solid electrolytic layer had been formed was washed with waterand dried. Then, a carbon paste and a silver paste were applied on thesolid electrolytic layer in this order. A cathode terminal made fromaluminum was attached to the silver paste and an anode terminal madefrom aluminum was attached to a terminal-fixing portion (bare portion)of the sintered tantalum. Further, a main body of the resultantcapacitor excluding portions for taking out electric energy of thecathode and anode terminals was housed in an epoxy resin to fabricate asolid electrolytic capacitor A1 (first capacitor).

FIG. 1 is a schematic sectional view of the solid electrolytic capacitorA1 fabricated in this embodiment. The solid electrolytic capacitor A1shown comprises an anode (of sintered tantalum) 1, a dielectric film (oftantalum oxide) 2, a solid electrolytic layer 3, a carbon paste layer 4,a silver paste layer 5, a silver paste layer 6, a housing resin (epoxyresin) 7, an anode terminal 8, a cathode terminal 9, and so on. Thesurface of the anode 1 is roughened by electropolishing and thedielectric film 2 is formed on this roughened surface by the anodicoxidation. The solid electrolytic layer 3 is formed on the dielectricfilm 2 by the electrolytic polymerization. The solid electrolytic layer3 is connected with a cathode including the carbon paste layer 4, thesilver paste layer 5 and the silver paste layer 6. The anode terminal 8is connected with the anode 1 and the cathode terminal 9 is connectedwith the silver paste layer 6, so that electric energy stored in thesolid electrolytic capacitor A1 by charge can be taken out through theseterminals. The entire main body of the solid electrolytic capacitor A1is housed in the housing resin 7 with parts of the anode terminal 8 andthe cathode terminal 9 exposed.

EXAMPLE 2

A solid electrolytic capacitor A2 (first capacitor) was fabricated inthe same manner as in Example 1 except that an aqueous solutioncontaining 0.2M of pyrrole, 0.06M of sodium dodecylsulfonate and 0.003Mof nickel sulfate was used as the solution for electrolyticpolymerization instead of the aqueous solution containing 0.2M ofpyrrole, 0.06M of sodium dodecylsulfonate and 0.0003M of sulfuric acid.

EXAMPLE 3

A solid electrolytic capacitor A3 (first capacitor) was fabricated inthe same manner as in Example 1 except that an aqueous solutioncontaining 0.2M of pyrrole, 0.06M of sodium dodecylsulfonate and 0.003Mof zinc sulfate was used as the solution for polymerization solutioninstead of the aqueous solution containing 0.2M of pyrrole, 0.06M ofsodium dodecylsulfonate and 0.0003M of sulfuric acid.

EXAMPLE 4

A solid electrolytic capacitor A4 (first capacitor) was fabricated inthe same manner as in Example 1 except that an aqueous solutioncontaining 0.2M of N-methylpyrrole, 0.06M of sodium dodecylsulfonate and0.0003M of sulfuric acid was used as the solution for electrolyticpolymerization instead of the aqueous solution containing 0.2M ofpyrrole, 0.06M of sodium dodecylsulfonate and 0.0003M of sulfuric acid.

EXAMPLE 5

A solid electrolytic capacitor A5 (first capacitor) was fabricated inthe same manner as in Example 1 except that an aqueous solutioncontaining 0.2M of thiophene, 0.06M of sodium dodecylsulfonate and0.0003M of sulfuric acid was used as the solution for electrolyticpolymerization instead of the aqueous solution containing 0.2M ofpyrrole, 0.06M of sodium dodecylsulfonate and 0.0003M of sulfuric acid.

EXAMPLE 6

A solid electrolytic capacitor A6 (first capacitor) was fabricated inthe same manner as in Example 1 except that an aqueous solutioncontaining 0.2M of N-methylthiophene, 0.06M of sodium dodecylsulfonateand 0.0003M of sulfuric acid was used as the solution for electrolyticpolymerization instead of the aqueous solution containing 0.2M ofpyrrole, 0.06M of sodium dodecylsulfonate and 0.0003M of sulfuric acid.

EXAMPLE 7

A solid electrolytic capacitor A7 (second capacitor) was fabricated inthe same manner as in Example 1 except that an aqueous solutioncontaining 0.2M of pyrrole and 0.06M of sodium dodecylsulfonate was usedas the solution for electrolytic polymerization instead of the aqueoussolution containing 0.2M of pyrrole, 0.06M of sodium dodecylsulfonateand 0.0003M of sulfuric acid.

EXAMPLE 8

A solid electrolytic capacitor A8 (second capacitor) was fabricated inthe same manner as in Example 1 except that an aqueous solutioncontaining 0.2M of N-methylpyrrole and 0.06M of sodium dodecylsulfonatewas used as the solution for electrolytic polymerization instead of theaqueous solution containing 0.2M of pyrrole, 0.06M of sodiumdodecylsulfonate and 0.0003M of sulfuric acid.

EXAMPLE 9

A solid electrolytic capacitor A9 (second capacitor) was fabricated inthe same manner as in Example 1 except that an aqueous solutioncontaining 0.2M of thiophene and 0.06M of sodium dodecylsulfonate wasused as the solution for electrolytic polymerization instead of theaqueous solution containing 0.2M of pyrrole, 0.06M of sodiumdodecylsulfonate and 0.0003M of sulfuric acid.

EXAMPLE 10

A solid electrolytic capacitor A10 (second capacitor) was fabricated inthe same manner as in Example 1 except that an aqueous solutioncontaining 0.2M of N-methylthiophene and 0.06M of sodiumdodecylsulfonate was used as the solution for electrolyticpolymerization instead of the aqueous solution containing 0.2M ofpyrrole, 0.06M of sodium dodecylsulfonate and 0.0003M of sulfuric acid.

Comparative Example 1

A comparative capacitor B1 was fabricated in the same manner as inExample 1 except that an aqueous solution containing 0.2M of pyrrole and0.06M of sodium borofluoride was used as the solution for electrolyticpolymerization instead of the aqueous solution containing 0.2M ofpyrrole, 0.06M of sodium dodecylsulfonate and 0.0003M of sulfuric acid.

Comparative Example 2

A comparative capacitor B2 was fabricated in the same manner as inExample 1 except that an aqueous solution containing 0.2M of pyrrole and0.06M of sodium perchlorate was used as the solution for electrolyticpolymerization instead of the aqueous solution containing 0.2M ofpyrrole, 0.06M of sodium dodecylsulfonate and 0.0003M of sulfuric acid.

Comparative Example 3

A comparative capacitor B3 was fabricated in the same manner as inExample 1 except that an aqueous solution containing 0.2M of pyrrole and0.06M of sodium dodecylbenzenesul fonate was used as the solution forelectrolytic polymerization instead of the aqueous solution containing0.2M of pyrrole, 0.06M of sodium dodecylsulfonate and 0.0003M ofsulfuric acid.

Comparative Example 4

A comparative capacitor B4 was fabricated in the same manner as inExample 1 except that an aqueous solution containing 0.2M of pyrrole and0.06M of sodium naphthalene sulfonate was used as the solution forelectrolytic polymerization instead of the aqueous solution containing0.2M of pyrrole, 0.06M of sodium dodecylsulfonate and 0.0003M ofsulfuric acid.

Capacitance, Capacitance Lowering Ratio and Impedance After Aging

The capacitance of the capacitors A1-A10 of the invention and thecomparative capacitors B1-B4 were measured at 120 Hz. Then, afterkeeping the capacitors at 200° C. for 3 hours for aging, the capacitanceat 120 Hz and impedance at 100 kHz were measured. Table 1 shows thecapacitance at 120 Hz after aging, the capacitance lowering ratiodefined by the following formula (1) and the impedance at 100 kHz afterthe aging of each capacitor:

Capacitance lowering ratio (%)=[(C1−C2)/C1]×100  (1)

wherein C1 indicates the capacitance at 120 Hz before aging and C2indicates the capacitance at 120 Hz after aging.

TABLE 1 Solid elec- Capacitance trolytic Capacitance lowering ratioImpedance capacitor (μF) (%) (mΩ) A1  150 0.7 30 A2  153 0.7 29 A3  1510.7 30 A4  152 0.8 40 A5  159 1.2 25 A6  160 1.5 21 A7  160 0.8 20 A8 162 0.9 20 A9  161 1.0 25 A10 160 1.2 25 B1  140 7.8 300 B2  140 9.7 330B3  150 3.0 90 B4  150 2.9 89

As shown in Table 1, the capacitors A1-A10 of the present invention havelarger capacitance, smaller capacitance lowering ratios and lowerimpedance than the comparative capacitors B1-B4.

Tantalum was used as the material for the anode in Examples 1-10, butalso when aluminum is used as the material for the anode, a solidelectrolytic capacitor having large capacitance and exhibiting lowimpedance in a high frequency region can be similarly obtained.

Although sodium dodecylsulfonate was used in Examples 1-10, any othersodium alkyl sulfonate can be used instead. Furthermore, it is possibleto use any metallic salt other than sodium salt, or an ammonium salt(such as quaternary ammonium salt) can be used instead of sodium salt.

Although water was used as a solvent for the solution for electrolyticpolymerization in Examples 1-10, any solvent can be used as far as theheterocyclic monomer and the dopant can be dissolved therein.

INDUSTRIAL APPLICABILITY

A solid electrolytic capacitor having large capacitance and exhibitinglow impedance in a high frequency region is provided.

What is claimed is:
 1. A solid electrolytic capacitor comprising ananode, a dielectric film formed on the anode by anodic oxidation, asolid electrolytic layer formed on the dielectric film, and a cathodeconnected with the solid electrolytic layer, the solid electrolyticlayer being made from a conducting polymer obtained by doping, withalkyl sulfonic acid ions, a polymer including a heterocyclic monomerunit represented by the following general formula as a repeating unit ina ratio of one of the alkyl sulfonic acid ions per 2 through 5heterocyclic monomer units represented by the general formula:

wherein R¹ and R² independently indicate an alkyl group or H, and Xindicates S or NR³ in which R³ indicates an alkyl group or H.
 2. A solidelectrolytic capacitor comprising an anode, a dielectric film formed onthe anode by anodic oxidation, a solid electrolytic layer formed on thedielectric film, and a cathode connected with the solid electrolyticlayer, the solid electrolytic layer being made from a conducting polymerobtained by doping, with alkyl sulfonic acid ions and sulfuric acidions, a polymer including a heterocyclic monomer unit represented by thefollowing general formula as a repeating unit:

wherein R¹ and R² independently indicate an alkyl group or H, and Xindicates S or NR³ in which R³ indicates an alkyl group or H.
 3. Thesolid electrolytic capacitor according to claim 2, wherein the polymeris doped in a ratio of one of alkyl sulfonic acid ions and sulfuric acidions per 2 through 5 heterocyclic monomer units represented by thegeneral formula.
 4. The solid electrolytic capacitor according to any ofclaim 1, 2 or 3, wherein the anode and the cathode are made fromaluminum or tantalum.